Servo systems for assisting in seeks and fine positioning of reading/recording heads for data storage devices are well known. For storage devices employing rotating media, such as hard disk drives, these servo systems generally involve the reading of certain data patterns (sometimes called “servo data”) recorded in one or more places on a hard disk surface which a head traverses. A not insignificant aspect of hard disk drive performance is access time, sometimes considered as the time it takes a head to go from one track to another on a disk surface. The shorter the access time, the better the performance.
When seeking to a particular track, the head will cross one or more tracks, and may read the addresses of tracks that the head traverses during the seek. In order for the head to arrive as quickly as possible at the correct track, it is desirable for the head to know what tracks it is traversing, at least in a relative sense, so that the head positioning system can accelerate or decelerate the head appropriately. If a head reads a track address inaccurately, a seek operation can take longer.
During a seek operation (such as those described above), a type of error known as inter-track interference is likely to occur where two adjacent addresses differ. To counter inter-track interference, Gray codes are usually used to ensure that adjacent track addresses on a disk surface differ in one and only one position. However, while Gray coded track addresses help to limit track ambiguity to one track during an offtrack condition, this type of coding does not permit correction of bit errors.
Also during a seek operation, it is practically inevitable that read back servo signals will contain noise that causes bit errors outside the position where two adjacent tracks differ. It would be desirable to provide a way of coding track addresses on data storage media so that the head positioning system can read the addresses accurately, and in particular, be able to correct at least certain types of bit errors that can occur during the reading process.